Computational studies of the relation between bond strength and QTAIM properties in molecular actinide compounds

In the development of the reprocessing of spent nuclear fuels, liquid extraction with ligands designed to selectively chelate minor actinides (MAs) in the presence of other cations is required. Extractants based on nitrogen donor ligands of the 2,6-bis(triazinyl)-pyridine (BTP) family can show high separation factors for the MAs from lanthanide fission products such as europium, and are also both radiation and low pH tolerant. In this PhD thesis, density functional theory (DFT) and the quantum theory of atom-in-molecules (QTAIM) are used to investigate the nature of actinide-nitrogen bonding in order to provide enhanced understanding of the selectivity of BTP and related ligands for the MAs. Several different DFT methods are initially benchmarked by calculating ionisation energies and bond dissociation energies of actinide oxides (for which high-level ab initio data are available in the literature). Subsequently, a series of calculations have been performed on some simple actinide complexes with one or three nitrogen-based ligands. QTAIM metrics are used to describe the relative roles of covalency and ionicity in the An-N bonding, and strong correlation is found between bond strength and partial charge difference of the actinide cations on compound formation. De Sahb et al. have proposed that the chemical properties of BTP and other polyazine-based ligands should reflect the contribution of their component single azine donor groups; to probe this, an investigation of the interaction energies of actinide-bisazines, lanthanide-bisazine and complexes of the azine components has been carried out. Strong correlation between the interaction energies of M-bisazines and the azine ligand components has been found. The interaction energy of a bisazine constructed from two of the same azine groups is shown to be a better indicator to the binding strength of bisazine ligands than the interaction energy of individual azine groups.